1. Field of the Invention
The present invention relates to a film bulk acoustic resonator (FBAR) and a method for manufacturing the same.
2. Description of the Related Art
As mobile communication apparatuses such as a mobile telephone are rapidly becoming more popular, there is an increased demand for a miniature light-weight filter for use therein.
In this regard, a film bulk acoustic resonator (FBAR) is known as a means suitable for implementing the miniature light-weight filter, and it is possible to make the FBAR in large quantities with minimum cost. In addition, it is possible to achieve a high quality factor which is a primary characteristic of the filter. Also, the FBAR can be used in a micro frequency band, and has an advantage of being able to operate in a PCS (Personal Communication System) and a DCS (Digital Cordless System) band.
Generally, the FBAR element is made by laminating in turn a lower electrode, a piezoelectric layer, and an upper electrode on a substrate. A principle of operation of the FBAR, is that electric energy applied to an electrode introduces an electric field varying in time in the piezoelectric layer. The electric field generates a Bulk Acoustic Wave in the same direction as a vibration direction of the resonance part in the piezoelectric layer, to produce resonance.
FIG. 1A is a cross sectional view of a bragg reflector type FBAR. Referring to FIG. 1A, the bragg reflector type FBAR comprises a substrate 10, a reflection layer 11, a lower electrode 12, a piezoelectric layer 13 and an upper electrode 14. In the bragg reflector type FBAR, an elastic wave generated in the piezoelectric layer 13 cannot be transmitted in a direction toward the substrate, and the elastic wave is entirely reflected by the reflection layer 11 to generate an effective resonance. A manufacturing process thereof comprises a step of forming the reflection layer 11 by depositing highly elastic impedance substances on the substrate 10 layer upon layer, and a step of forming a resonance part on the reflection layer 11 by stacking, in turn, the lower electrode 12, the piezoelectric layer 13 and the upper electrode 14. The bragg reflector type FBAR has a strong structure and is not subject to stress by bending. However, this type of FBAR has a disadvantage in that it is difficult to form more than four reflection layers having an exact thickness needed for full reflection. Also, much time and cost are needed for manufacturing.
Accordingly, research has been conducted relating to an air gap type FBAR, and in this type of FBAR, the substrate and the resonance part are isolated by an air gap instead of a reflection layer. FIG. 1B is a cross sectional diagram showing the structure of a conventional air gap type FBAR.
In the FBAR shown in FIG. 1B, an air gap 21 is formed under the resonance part, where the lower electrode 23, the piezoelectric layer 24 and the upper electrode 25 are laminated in turn, to separate the resonance part and the substrate 20. In a manufacturing process thereof, a sacrificial layer (not shown) is deposited on the substrate 20, and after a patterning process, the sacrificial layer remains on a predetermined area on the substrate. Next, an insulating layer 22 is deposited on the sacrificial layer and the substrate 40, and the lower electrode 23, the piezoelectric layer 24 and the upper electrode 25 are formed in turn to make the resonance part. The insulating film 22 functions as a membrane layer which supports the resonance part. Finally, the sacrificial layer is removed to form the air gap 21. In this regard, a via hole is formed which connects an exterior of the element to the sacrificial layer, and an etching liquid is injected through the hole to remove the sacrificial layer, to thereby form an air gap 21 in the place of the sacrificial layer. Meanwhile, in reference to U.S. Pat. No. 6,355,498, when the air gap type FBAR shown in FIG. 1B is manufactured, a substance which prevents etching is used to adjust the size and position of the air gap. However, the above manufacturing method requires a sacrificial layer, thereby making the manufacturing process complex. In addition, the via-hole should be formed on the membrane layer, causing a limitation in filter design. In addition, the etching process using the via-hole formed very near the resonance part may cause chemical damage to the resonator. Meanwhile, considering the problem of existing FBAR elements, an air gap type FBAR has been developed, which is made by depositing a lower electrode, a piezoelectric layer and an upper electrode on a substrate to form a resonance part and then etching a lower part of the substrate to form an air gap. However, in this type of air gap FBAR, the process of etching the lower part of the substrate may damage the resonance part, thereby deteriorating resonance characteristics.